Add deterministic process graph liveness validation

[dkharlanau]

Fixes #32.

Adds graph-liveness analysis on top of existing reachability validation:

• reachable terminal detection instead of accepting an unrelated unreachable terminal;
• reverse reachability to identify every reachable step that cannot terminate;
• deterministic strongly-connected-component detection for trapped cycles while allowing retry/rework loops that have an exit;
• structural error for type: end with outgoing transitions;
• backward-compatible warning for reachable non-end implicit terminals;
• decision warning for fewer than two outgoing branches;
• parallel split/join semantic warning when the node has neither multiple inputs nor multiple outputs.

Regression tests cover terminating loops, trapped cycles with an unreachable terminal, end-with-outgoing, implicit terminals, decision branch cardinality and parallel split/join behavior. The specification and architecture docs now define the liveness boundary explicitly.

The first PR CI pass exposed one existing fixture problem rather than a validator defect: examples/adapters/process-manifest.csv described a linear manager approval with one outgoing edge as a decision gateway. Because that CSV shape does not model branches and the step is operationally a human approval task, the fixture is corrected to user_task instead of weakening the new gateway rule.

The validator remains static: liveness means the declared graph has terminating paths, not that external runtime work is guaranteed to finish.

[github-actions]

Process as Code — PR impact

No changed process contracts detected.